Infants born prematurely commonly experience a fall in hemoglobin in the first few months of life. This decrease in hemoglobin exceeds that of normal term infants by several g/dl. The factors governing this fall in hemoglobin and the physiologic consequences of this alteration in oxygen carrying capacity are not understood. Decisions concerning transfusion are often made empirically rather than on the basis of sound physiologic data. The purpose of this research is to correlate those factors which are considered important in the regulation of tissue oxygen with the premature infant's responses to these changes. In addition, the in vitro erythroid colony responses of premature infants to erythropoietin should be fully investigated to determine if the anemia of prematurity is in anyway related to a failure of appropriate responses or to the presence of erythropoiesis inhibiting factors. Infants weighing less than 1500 g at birth will be followed throughout their nursery stay. Studies will be performed every two weeks to determine tissue oxygen availability (arterial blood gas analysis, cardiac output, hemoglobin concentration and P50 determinations) and the response to this oxygen availability and the actual need for oxygen (oxygen consumption, erythropoietin levels and biochemical evidence of tissue hypoxia). In vitro erythropoietin dose related erythroid colony responses will be compared to specimens obtained from older age group controls. It is anticipated that a true physiologic definition of anemia will be able to derived from this study.